Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as, for example, the oxides of nitrogen (NOX). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOX emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR).
SCR is a process where gaseous or liquid reductant (most commonly a solution of urea solid and water) is added to the exhaust gas stream of an engine and is adsorbed onto a catalyst. The reductant reacts with NOX in the exhaust gas to form H2O and N2, which can be safely released to the atmosphere. Although SCR can be an effective method for reducing NOX, it can be difficult to ensure that enough reductant has been injected to adequately reduce the amount of NOX present within the exhaust gas stream, without unnecessarily wasting reductant and releasing unused reductant or byproducts thereof to the atmosphere.
One attempt to regulate the injection of reductant is described in U.S. Pat. No. 6,361,754 (the '754 patent) issued to Peter-Hoblyn et al. on Mar. 26, 2002. The '754 patent discloses an exhaust system for reducing NOX emissions from an internal combustion engine. The exhaust system includes an exhaust pipe of an engine, and an SCR catalyst located within the exhaust pipe. The exhaust system also includes a tank of urea, a pump that pressurizes the urea, a valve that meters the pressurized urea, and a nozzle that injects the metered and pressurized urea into the exhaust pipe upstream of the SCR catalyst.
The injection of urea into the exhaust by the system of the '754 patent is regulated by a feedforward controller in response to a number of measured parameters, including: fuel flow, throttle setting, engine speed, rack setting, intake air temperature, barometric pressure, intake air humidity, and exhaust gas temperature. In addition, to the extent the sensors are available, trim or feedback control is provided based on residual levels of gas species following the SCR catalyst, e.g., the level of NOX, HC, or CO. If desired, feedback control can also be employed to trim the system in response to residual levels of ammonia.
Although perhaps somewhat effective at controlling reductant injections, the exhaust system of the '754 patent may fail to consider some critical factors affecting the reduction of NOX within the SCR catalyst. For example, a temperature of the SCR catalyst can have a great effect on the reduction efficiency of the catalyst. Similarly, a flow rate of exhaust through the catalyst (and/or an associated dwell time within the catalyst) can also have a significant impact on the reduction of NOX. Because the exhaust system of the '754 patent does not take these parameters into consideration, the system may still allow sub-optimal reductant injections to occur. And, although the exhaust system of the '754 patent may trim reductant injections based on residual levels of some gas species (NH3) following the SCR catalyst, there may be more efficient ways to affect those levels and additional triggers for affecting those levels.
The system of the present disclosure solves one or more of the problems set forth above and/or other problems.